In order to calculate the relative value of the amounts of grain mentioned in the various sources as provided by the thematic authorities for various expeditions, as well as the numbers of animals required to transport a specific quantity of supplies for men and animals over a specific period of time, the value of the measures which are used must first be established, and this is still to a degree problematic, in spite of the work of several specialist scholars. Such calculations depend on a range of variables which have aroused a great deal of disagreement. The measure used in the majority of texts dealing with grain from the middle Byzantine period is the modios, but since there were several different modioi, and since the relationship between the various modioi and other measures, such as the litra (the Roman pound), on the one hand and, on the other, late ancient values for weight and volume such as the artaba remain unclear, it is not possible simply to read off the values from the texts in question.
Most of the information on the relationship between the artaba and the modios for the late Roman period comes from Egyptian documents, and although there are some difficulties (because of the variety of equivalences given in different localities), an equivalence of 4.5 basic (or “Roman”) modioi or 3.3 modioi xystoi to the artaba can be derived from fiscal documents of the fifth and sixth centuries; nevertheless there are several other equivalences, depending on which of the various modioi are meant. 1 The issue is complicated by the fact that the modios is a measure of volume or capacity, and that in consequence it is difficult to extrapolate a weight in order to calculate the results of any conversion from grain to flour and thence through the baking process to bread. Equivalences between volume and weight must therefore remain averages, the more so since different types of grains weighed differently and were of different density, so that a modios of barley is by no means the same quantity, by weight or by product, as a modios of wheat. 2 Matters are further complicated by other factors, in particular the variable value of the